Pre-implantation genetic diagnosis (PGD)
PGD (Pre-implantation genetic diagnosis)
Pre-implantation genetic testing (PGD) is a technique done to identify genetic defects before pregnancy in embryos produced through IVF( in vitro fertilization). Pre-implantation genetic diagnosis is a genetic test on the cells taken from embryos, to select the best embryos to attain pregnancy or to avoid a genetic disorder that a couple is at risk. This test is performed to determine if the embryo carries the genetic abnormality when one or both the parents have a known genetic abnormality.
Indications for Pre-implantation Genetic Diagnosis
Pre-implantation genetic diagnosis (PGD) is highly recommended when a couple of known genetic abnormality is at risk of transmitting it to their children. The risk of inheriting the genetic abnormality and late pregnancy termination is reduced as only the healthy and normal embryos are transferred into the mother’s uterus.
Primary candidates for PGD
These include the following:
• A family history of X-linked disorders (A family history of X-linked disease has a 25% risk of having an affected embryo [half of the male embryos].)
• Chromosomal translocations in couples which may lead to Implantation failure, recurrent pregnancy loss, or mental or physical problems in offspring,
• Carriers of autosomal recessive diseases (The risk of an embryo being affected is 25% in carriers of the autosomal recessive disease.)
• Carriers of autosomal dominant diseases (the risk of an embryo being affected is 50% carriers of the autosomal dominant disease.)
Conditions that are diagnosed using PGD
PGD should be done for three major groups of disease:
• Sex-linked disorders
• Single gene defects
• Chromosomal disorders.
The process of PGD
The procedure of IVF has to be done to have embryos to biopsy for PGD/PGS. After fertilization of the egg with sperm, the embryos are developed to a cleavage stage, After egg retrieval on the third day, one blastomere is removed from the developing embryo for genetic evaluation of the embryo. PCR, FISH, or comparative genomic hybridization (CGH) is performed for genetic evaluation. The normal or the non-affected embryos are then transferred into the uterus for subsequent implantation and pregnancy.
Advantages of PGD
PGD can help eliminate some genetic diseases like Tay-Sachs disease, cystic fibrosis, Huntington disease, X-linked dystrophies in the future. For several genetic diseases complete cures don’t seem to be likely to be found; therefore, preventing the disease is preferable to awaiting a possible cure to eventually become available. Furthermore, available treatments often have multiple adverse effects.
Prenatal testing for genetic diseases is currently performed through amniocentesis or chorionic villus testing (CVS) which is done when the foetus is aged 10-16 weeks are the pre-natal tests performed for genetic diseases. If the findings reveal a genetically defective foetus then choices available to parents are to have a baby with a genetic disorder or to undergo a pregnancy termination which is a difficult and traumatic decision to make especially in advanced pregnancy. Since PGD is performed before pregnancy begins eliminates this difficult decision.
In the past, persons with a genetic disorder or who know that they are carriers chose not to have children to avoid the risk of passing on the disease to future generations. Now, child-free of their particular disease is possible with PGD.
Is embryo biopsy and PGD safe?
Yes. Data from many years of PGD in animals and several other hundred thousand live births in humans indicate that PGD doesn’t result in an increase in birth defects over that of the general population. There is no evidence for a harmful effect of the process on growth or neurological development in children born after PGD. In embryos where chromosomal PGD testing is performed, one can expect fewer pregnancies ending in miscarriages because of chromosomal disorders since most abnormalities are identified before the transfer of the embryos to the uterus. Removal of the blastomere cell of the early embryo does not alter the ability of that embryo to develop into a complete, normal pregnancy.